Rinse berries under cold running water. Allow excess water to drain off.
Prepare strawberries by removing stems and cutting them into small pieces.
Place the chopped strawberries, raspberries and sugar in a saucepan.
Cook over medium heat, stirring frequently to combine.
Cook until berries release their juices and the mixture comes to a boil, about 8-10 minutes, stirring frequently.
Reduce heat and simmer mixture for 10 minutes.
Use the back of a spatula to smash the strawberry pieces on the side of the pan to break them down.
Continue to simmer until mixture thickens.
Use a spoon to skim off and discard any foam that forms on the surface.
Add lemon juice and simmer for an additional 3-4 minutes.
Remove from heat and allow the mixture to cool for 15 minutes. Mixture will thicken further as it cools.
Spoon cooled mixture into clean glass jars and top with lids.
Allow preserves to cool to room temperature.
Once cooled, store jars in the refrigerator for up to 3 months.